Carbon nanotubes are a hollow carbon substance in which a graphite sheet (that is, a graphene sheet or carbon atomic plane with a graphite structure) is rolled into a tubular shape. The diameters of these tubes are on the nanometer scale, and the walls are of graphitic structure. These carbon nanotubes were discovered in 1991 by Dr. Sumio Iijima. Carbon nanotubes in which the wall structure consists of a single graphite sheet closed in a tubular shape are called single-walled carbon nanotubes, while those consisting of a plurality of graphite sheets each closed into a tubular shape and nested one within the other are called nested multi-walled nanotubes.
Tubes that are similar to, but different in carbon wall structure from, the nested multi-walled carbon nanotubes, have been reported, in which the graphite wall structure is in a scroll form.
In an effort to improve the electrical characteristics in the field of electrical conductors and the like and the magnetic characteristics, there have been attempts in recent years to encapsulate a metal within the internal spaces defined by the tube walls of these carbon nanotubes (hereinafter sometimes referred to as “CNTs”) and so forth.
For example, Japanese Patent No. 2,546,114 discloses a foreign substance-containing carbon nanotube in which a substance other than carbon, such as a metal, is encapsulated in the cavity at the center of a nested multi-walled carbon nanotube. This foreign substance-containing carbon nanotube is prepared by vapor depositing a substance other than carbon at the end of a nested multi-walled carbon nanotube closed by a cap, either during or after the removal of the cap, and introducing the substance by thermal diffusion into the cavity located at the center of the carbon nanotube from the end of the carbon nanotube.
Japanese Unexamined Patent Publication No. 1997-142819 discloses carbon tubes each composed of a carbon nanotube having a diameter of 10 nm to 1 μm and a length of 1 to 100 μm and a foreign substance contained in the carbon nanotube. These foreign substance-containing carbon nanotubes are prepared using an inorganic substance having substantially straight channels as a template, either by coating the channel walls with an organic substance and carbonizing the organic substance by heating, or by subjecting a gaseous hydrocarbon to vapor phase carbonization inside the channels so as to deposit a thin film of carbon, thereby forming carbon tubes, and then bringing a solution or a melt of said foreign substance into contact with the tubes to insert the foreign substance into the cavities of the carbon nanotubes (and removing the inorganic substance by dissolving it before or after the insertion).
Japanese Unexamined Patent Publication No.2000-204471 discloses minute metal wires each composed of a wire material having a diameter of 1 to 100 nm and having a major axis length to diameter ratio of at least 50, and more particularly discloses a minute metal wire covered with a tube made of carbon. This minute metal wire covered with a carbon tube is prepared by substantially the same process as that disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 1997-142819. The process comprises the first step of forming, in an inorganic substance having substantially linear channels, tubular carbon on the channel walls, and the second step of depositing metal in the interior of this tubular carbon.
However, the above-mentioned prior art processes require at least two steps of first forming a carbon tube and then inserting a foreign substance, so that the processes are complicated in terms of management and control of the steps, and entails low productivity. Furthermore, the preparation processes disclosed in the above-mentioned Japanese Unexamined Patent Publications Nos. 1997-142819 and 2000-204471 require a step of removing the inorganic substance used as a template by dissolving it.
Also, so far no process has been developed with which a composite comprising a metal, and particularly iron or an iron compound, contained in the internal space defined by the carbon wall of a tubular carbon material, such as carbon nanotube, can be obtained in amounts over the milligram scale. Consequently, practical research has not been done on such carbon-metal composites in which a metal or the like is contained in the internal space of a tubular carbon material.
The primary object of the present invention is to provide a composite in which iron or an iron compound is contained within a considerable portion of the internal space of a carbon tube, a carbonaceous material containing such composites, and processes for preparing the same.